To determine whether axons exposed to inflammation become vulnerable to damage by CD8+ cytotoxic T cells (CTLs) responsible for in vivo axon injury.

Mature, healthy neurons do not express major histocompatibility complex class I (MHC-I) molecules necessary for canonical CTL-mediated injury. However, inflammation drives neuronal MHC-I expression in vitro. Neurons expressing peptides recognized by transgenic CTLs or virally infected neurons are damaged by CTLs in some models. Further, CD8+ cells are necessary and sufficient for axon injury in the Theiler’s virus (TMEV) model of multiple sclerosis (MS). We sought to determine whether uninfected, wild type axons exposed to inflammation would become vulnerable to injury by CTLs isolated from the CNS of TMEV infected mice.

Mouse cortical neurons were cultured in microfluidic chambers physically and fluidically separating axons and cell bodies. Axons were exposed to interferon gamma (IFN-γ) for 72 hours and MHC-I expression was assessed via RT-PCR and immunostaining. IFN-γ treated axons were incubated with spinal cord infiltrating leukocytes isolated from the spinal cords of TMEV infected mice and axon injury was assessed by immunostaining for neurofilament.

Expression of pan-MHC-I, H-2K and H-2D RNA transcripts increased by 15, 5.3, and 12.8 fold (respectively) following axon exposure to IFN-γ. Immunostaining for MHC-I was positive only in axons exposed to IFN-γ. Axons treated with IFN-γ and incubated with CD8+ enriched T cells isolated from the CNS of TMEV infected mice showed loss of axon fiber density and increased axon breakage. Damage did not occur in axons not exposed to IFN-γ or treated with vehicle.

Exposing axons to IFN-γ results in MHC-I expression and renders axons vulnerable to damage by CNS infiltrating lymphocytes previously implicated in axon injury. These results suggest a plausible mechanism by which CNS axons exposed to an inflammatory milieu, such as an MS lesion, may be damaged by CTLs.